An organic EL device comprises a thin film containing a fluorescent organic compound interleaved between an electron injecting electrode (cathode) and a hole injecting electrode (anode), and emits light making use of light emissions (fluorescence and phosphorescence) upon deactivation of excitons which are generated by injecting electrons and holes in the thin film for their recombination.
Features of the organic EL device are that surface light emission of high luminance of the order of hundreds candelas/m2 to scores of thousands of candelas/m2 is achievable at a low voltage of up to 10 V, and blue to red light emissions are achievable by selecting the type of fluorescent material.
On the other hand, problems with the organic EL device are that its light emission life is short, and its storage robustness and reliability are low for the following reasons.
(1) Physical Changes of Organic Compound
An inhomogeneous organic thin film interface caused as by the growth of crystal domains is responsible for a drop of the ability of the device to inject holes, short-circuiting, and dielectric breakdown. Especially when a low-molecular compound having a molecular weight of 500 or less is used, crystal grains manifest themselves and grow greatly, resulting in a strikingly reduced film property. When an organic thin film interface with ITO or the like is roughened, too, crystal grains manifest themselves and grow greatly, resulting in a light emission efficiency drop and current leakage which eventually lead to non-emission. This is also a leading cause of local non-emitting spots, i.e., dark spots.
(2) Oxidation and Delamination of Electron Injecting Electrode
To facilitate injection of electrons, for instance, Na, K, Li, Mg, Ca, and Al has so far used as a metal having a low work function. However, when these metals react with atmospheric moisture or oxygen or the organic layer peels off the electron injecting electrode, it is impossible to inject holes. Especially when a high-molecular compound is formed into film as by spin coating, solvent residues after film formation, moisture and decomposed products accelerate the oxidation reaction of the electrode, and causes the delamination of the electrode, resulting in local non-emitting spots.
(3) Low Light Emission Efficiency and Generation of Much Heat
The generation of heat is unavoidable because a current is passed through the organic compound and so the organic compound must be placed in a high field strength. The heat then gives rise to the melting, crystallization, and thermal decomposition of the organic compound, resulting in a deterioration and breakdown of the device.
(4) Photochemical, and Electrochemical Changes of Organic Compound Layer
Upon passing a current through the organic material, the organic material degrades, resulting in defects such as current or exciton traps. These defects in turn cause a deterioration of the device such as a driving voltage increase or a luminance drop.
Practical light emitting devices are used in various environments. Especially when such a device is used in high-temperature environments, the quality of display images drops or the device breaks down because of the crystallization or physical change of the organic compound or the rearrangement, i.e., migration, dispersion, etc. of the organic molecules.
A hole or electron injecting electrode interface that is an interface between an organic material and an inorganic material, especially the hole injecting electrode interface has a great influence on the film property of the organic material layer during film formation. In some cases, several problems arise; an inhomogeneous organic layer is formed on the hole injecting electrode, and no good interface can be formed.
For this reason, it has so far been reported to use on the hole injecting electrode interface in an organic EL device materials such as phthalocyanine, polyphenylene-vinylene, evaporated polythiophene film, and amine polymer. However, it is found that the use of phthalocyanine (U.S. Pat. No. 4,720,432 or JP-A 63-295695) is not preferable because it yields a device which can be well operated in an initial state, but dark spots, light emission variations, etc. manifest themselves while it is operated over an extended period. This is because the phthalocyanine accelerates the crystallization of a material placed thereon due to its own microcrystalline nature. For polyphenylene-vinylene, it is required to use a wet process such as spin coating wherein atmospheric impurities such as moisture are entrained therein, or ionic impurities such as leaving groups are entrained therein upon conversion from its precursor. Thus, the oxidation of the electrode proceeds rapidly, causing a striking luminance drop or a noticeable driving voltage increase.
Problems with the evaporated polythiophene film are that the reproducibility of fabrication of good devices is low due to large variations in the degree of polymerization of polythiophene and large fluctuations in polythiophene during evaporation, and the surface of ITO cannot be fully denatured due to difficulty involved in making polythiophene thick because the polythiophene itself has light absorption in a visible light region. Regarding amine polymers, for instance, dendrimer materials (JP-A 4-308688), tetraamine materials (U.S. Pat. No. 439,627) and triamine materials (JP-A 8-193191) have been reported. However, it is found that sufficient heat resistance is not obtained, and that a homogeneous and stable film cannot be obtained on a hole injecting electrode during high-temperature storage.